Device for coating substrates



July 7, 1964 G. A. JENSEN DEVICE FOR COATING SUBSTRATES Filed Sept. 8.1961 JENSEN INVENTOR.

ATTORNEYS GERALD A United States Patent DEVICE FOR COATING SUBSTRATESGerald A. Jensen, Lowell, Mass., assignor to Avco Corporation,Cincinnati, Ohio, acorporation of Delaware Filed Sept. 8, 1961, Ser. No.136,812 11 Claims. (Cl. 219-76) This invention relates to devices forcoating substrates and, in particular, to are plasma spray-coatingdevices.

An electrical arc is a source of extremely high heat energy. A fluid,when interacted with an electric arc, ab-

sorbs heat from the arc and partially dissociates into free electronsand ions. The partially dissociated fluid is electrically neutral and isgenerally called a plasma. Eventually, when the plasma leaves thevicinity of the arc, the electrons and ions tend to recombine. Duringthe recombination process, heat is give up to the surroundingenvironment. The plasma thus acts as a source of heat energy.

An arc plasma spray-coating device of the type being considered in thisdiscussion is described in the Yenni Patent 2,982,845. It will be notedthat a rod of coating material is fed laterally into a plasma eflluentgenerated by an arc plasma generating device A. This approach has atleast two limitations. There is a tendency for the rod to melt unevenlyand to produce large particles of coating material. A poor coatinginevitably results. Large particles typically are not heatedsufficiently or accelerated to a high enough velocity to produce asuitable bond with a substrate; additionally, particle depositionvelocity will also be low since:

(1) The rod impedes the flow of the efiiuent; and

(2) Energy must be expended in changing the lateral movement of rodmaterial to a generally axial direction.

As may also be expected, the rod deflects the plasma efiiuent so thatrod particles have a lateral velocity component when leaving the plasmagenerator and therefore, tend to disperse quickly. g

A second suggested configuration for an arc plasma spray-coating devicecalls for the rod of coating material to be fed through a hollow centerelectrode-a hollow version of electrode 10 in FIGURE 1 of the Yennipatent,

for example-directly through the arc.

Experience has shown that the arc spot should remain stationary on thecenter electrode, generally the cathode electrode, for long life. On aring electrode, such as would result if the rod material were fedthrough the center of the center electrode, there is a tendency for thearc to move around. This type of operation increases the erosion rate ofthe center electrode.- Additionally, the

alternate heating and cooling of portions of the center electrode ringcauses severe heat stresses. In many cases,

.these stresses have been suflicient to cause the electrode to crack ordisintegrate.

An additional important limitation of feeding coating material throughthe arc is the tendency for this material to cause instability in thearc. Nozzle openings of arc plasma generating devices are small, 4"diameter being relatively large. It would be very difficult to a spray aA rod with a A" nozzle opening. Some of the rod material would adhere tothe nozzle and build up to a point where the flow of. the plasmaefiluent would be completely blocked. At the same time as the throatarea 3,140,380 Patented July 7, 1964 level required to operate the arcat a given power, it is desirable to have operating voltages as high aspossible which means operating at higher are chamber pressures. When thethroat'diameter is increased, the only way that the arc chamber pressurecan be kept at the desired value is to increase the rate of mass flow ofworking fluid. Manifestly, this is costly and inefiicient.

There are many advantages to spraying large rods, and /2 diameter rods,for example. Manifestly, the problems of maintaining proper areparameters with reasonable fluid mass flow increase sharply in severity.

It is an object of the invention to provide a device for coatingsubstrates which avoids the limitation and disadvantages of prior artdevices.

It is another object of the invention to provide an arc plasmaspray-coating device for coating substrates which comprises two or moreplasma effluents merging into a joint plasma eflluent into which acoating material is fed and reduced to substantially molten particlesand deposited on a substrate. 1

It is still another object of the invention to provide an arc plasmaspray-coating device which is reliable, operates with low fluid massflow, and does not affect the stability of the plasma generating means.

In accordance with the invention, a device for coating substratescomprises a plurality of separate arc devices for generating a pluralityof plasma effluents. The plasma effluents are directed to intercept at apoint and merge to form a joint plasma eflluent.

The are plasma spray-coating device also includes means for feeding arod of coating material into the joint plasma efl'luent.

In addition, provision may be incorporated in arc plasma spray-coatingdevice for supplying a secondary high velocity fluid stream foraccelerating the coating particles from the plasma effluent to a desireddeposition velocity.

The novel features that are considered characterstic of the inventionare set forth in the appended claims; the invention itself, however,both as to its organization and method of operation, together withadditional objects and advantages thereof, will best be understood fromthe following description of a specific embodiment when read inconjunction with the accompanying figure which is a plasma spray-coatingdevice embodying the principles of the present invention.

There is represented in this figure a device for coating substrates and,more particular, an arc plasma spray-coating device 10. The are plasmaspray-coating device 10 includes a first plasma generating means 11 anda second plasma generating means 12. Plasma. generators 11 and 12 arepreferably identical in construction and performance and are angularlydisplaced symmetrically with relation to an axis 13. I

Plasma generator 11 includes a center electrode 14 and a nozzleelectrode 16. The electrode 14 and the electrode 16 are disposed. in acoaxial relationship. Electrodes 14 and 16 are connected to an electricpower supply 18 as indicated. When plasma generator 11 is energized, an

are 19 is generated across electrodes 14 and'16. A fluid,

nected to an external gas supply means (not shown).

Such gas may be any suitable arc gas such as argon, nitrogen, hydrogen,or helium.

The gas flowing through the are 19 absorbs heat from the are 19 and isconverted into a plasma effluent 17 which leaves the plasma generator 11by way of the nozzle electrode 16.

As is obvious from FIGURE 1, plasma generator 12 1 =19 is identical toplasma generator 11. It includes a center electrode 23, a nozzleelectrode 24, an arc 26 and a gas supply passage 27. In a manner similarto that indicated above, plasma generator 12 produces a plasma effiuent28. Plasma effluent 28 merges with plasma efiluent 17 to form a jointplasma efiiuent 29.

The are plasma spray-coating device also includes means for supplyingcoating material to the joint plasma effluent 29 where such material isheated into minute molten or plastic particles 22 and accelerated to asubstrate surface (not shown). A passage 39 which is coaxial with axis13 and midway between the plasma generators 11 and 12 is provided in thearc plasma spray-coating device 10. A rod 34 formed from a desiredcoating material is inserted in the passage with one end projecting intothe joint plasma efiiuent 29 at or near the junction of plasma effluents17 and 28. The rod 34 is continuously fed by a feed mechanism shownschematically as rollers 36. Manifestly, passage 30 or a modificationthereof may be used to provide coating material in the form ofparticles.

The symmetrical relationship shown in FIGURE 1 between plasma generators11 and 12 and rod 34, in addition to locating the end of rod 34 at ornear the junction of the plasma efiluents l7 and 28 generate a pointedend from which a coating material is uniformly removed from the rod 34.In FIGURE 1 two plasma generators are shown and accordingly, the pointon rod 34 takes the form of a chisel point. Where three or more plasmagenerators are used, it is clear that the point will approach a conicalshape.

It will be noted that the coating material does not interfere with theoperation of the plasma generators 11 and 12. Their operating parameterssuch as voltage, current mass flow rate, etc. are determinedindependently of the material being used to coat a substrate. It willalso be noted the deposition of coating material in the arc plasma spraydevice 10 may be minimized since its exit aperture can be designedindependently of the plasma generators 11 and 12 to avoid coatingdeposition within the arc plasma spray-coating device 10. Theparticles22 leave rod 34 in substantially the same direction as the direction offlow of the plasma effiuent 29. Higher particle velocities are thusachieved.

In the event it is desired to provide an independent means foraccelerating the particles 22, the arc plasma spray-coating device 10includes means, in the form of passages 31 and 32, through which a blastof air or other suitable fluid may be directed into the joint plasmaefiiuent 29 to accelerate the particles 22. The aforementionedaccelerating means may also be used to prevent or minimize dispersion ofthe joint plasma effluent 29.

A coating material in the form of a rod is preferred over powderedcoating material, where a large surface is to be coated in a short time;i.e., where high deposition rates are desired. To transform a rod intosprayable particles requires a large amount of heat, much more than isrequired to spray powdered coating material. The heat must also beapplied uniformly for the reasons discussed heretofore. The use ofmultiple plasma generating devices and merging their plasma efiluentsmeets both the requirement of high heat and uniform heat distributionuniquely.

Obviously, since the rod is set centrally into the joint plasmaefliuent, heating over the entire surface of the rod is achieved.

Additionally, the use of the single high-heat source, a large arc plasmagenerator for example, is avoided. A single source is less efficient,more costly and requires more frequent replacement of components than aplurality of arc plasma generating units having an aggregate heat outputequaling the output of a single source.

The various features and advantages of the invention are thought to beclear from the foregoing description. Various other features andadvantages not specifically enumerated will undoubtedly occur to thoseversed in the art, as likewise will many variations and modifications ofthe preferred embodiment illustrated, all of which may be achievedwithout departing from the spirit and scope of the invention as definedby the following claims.

I claim:

1. A device for coating substrates comprising: means for generating afirst plasma efiluent; means for generating a second plasma effluentwhich merges with said first plasma effluent to form a joint plasmaeffiuent; and means for supplying coating material to the joint plasmaeffiuent.

2. A device for coating substrates as defined in claim 1 which includes,in addition, means for supplying a high velocity fluid stream to thejoint plasma efiluent for accelerating the emission of said coatingmaterial from said joint plasma effiuent.

3. A device for coating substrates as defined in claim. 1 in which saidmeans for generating said first and second plasma eflluent comprise arcplasma generating means.

4. A device for coating substrates comprising: a plurality of angularlyplaced means for generating a plurality of merging plasma effluents forforming a joint plasma effluent; and means for supplying coatingmaterial into said joint plasma eflluent.

5. A device for coating substrates as defined in claim 4 which includesin addition means for supplying a highvelocity fluid stream to saidjoint plasma effiuent for accelerating the emission of said coatingmaterial from said joint plasma effluent.

6. A device for coating substrates comprising: a pair of angularlydisplaced co-planar means for generating a pair of co-planar mergingplasma effluents for forming a joint plasma effiuent; and means forfeeding, in the plane defined by said pair of plasma eflluents, a rod ofcoating material to said joint plasma efiluent.

7. A device for coating substrates as defined in claim 6 in which saidrod bisects the angle separating said generating means.

8. A device for coating substrates as defined in claim 6 in which themeans for generating plasma comprises are plasma generating means.

9. A device for coating a substrate which uses coating material in theform of a rod comprising: nozzle means including an exit orifice alignedwith a central axis; a plurality of arc plasma generating means in anangularly spaced relationship with respect to the central axis forgenerating a plurality of plasma effiuents directed toward the exitorifice and merging adjacent to the exit orifice into a joint plasmaeffinent emerging from the exit orifice; and means for feeding the rodto the exit orifice and into the joint plasma effluent.

10. A device for coating a substrate which uses coating materials in theform of a rod comprising: nozzle means including an exit orifice alignedwith a central axis; a plurality of arc plasma generating means in anangularly spaced relationship with respect to the central axis forgenerating a plurality of merging plasma effiuents to form a jointplasma efiluent emerging from the exit orifice; means including a secondpassage aligned with the central axis through which the rod isinsertable; and means for continuously feeding the rod to the exitorifice and the joint plasma effluent at a predetermined rate togenerate sprayable particles.

11. A device for coating a substrate as defined in claim 10 whichincludes means for supplying a high velocity stream of fluid to thejoint plasma efiluent for accelerating the sprayable particles.

References Cited in the file of this patent UNITED STATES PATENTS1,037,979 Perkins Sept. 10, 1912 1,243,795 Apple Oct. 23, 1917 2,754,225Gfeller July 10, 1956 2,806,124 Gage Sept. 10, 1957 2,982,845 Yenni etal. May 2, 1961

1. A DEVICE VOR COATING SUBSTRATES COMPRISING: MEANS FOR GENERATING AFIRST PLASMA EFFLUENT: MEANS FOR GENERATING A SECOND PLASMA EFFLUENTWHICH MERGES WITH SAID FIRST PLASMA EFFLUENT TO FORM A JOINT PLASMAEFFLUENT: AND MEANS FOR SUPPLYING COATING MATERIAL TO THE JOINT PLASMAEFFLUENT.